The present invention relates to modulation amplifiers, and more particularly to a low-power, high-modulation-index amplifier suitable for use in battery-powered devices, e.g., an implantable battery-powered medical device.
Modulation is the process of varying some characteristic of one wave in accordance with another wave. In radio broadcasting, for example, some stations use amplitude modulation (AM), while other stations use frequency modulation (FM). In television, the video portion of the program is amplitude modulated and the audio portion is frequency modulation. In other types of transmissions, such as are used with satellite transmissions or transmissions to and from an implantable medical device, some sort of digital modulation is typically employed, e.g., pulse-amplitude, pulse-code, pulse-duration, pulse-frequency, pulse-position, or pulse-time modulation. These types of digital modulation are typically employed to convey binary bit information, e.g., strings of 1""s and 0""s, arranged in words and bytes.
A class of small implantable medical devices is known in the art that comprises tiny microstimulators and/or sensors. These tiny microstimulators or sensors, which are hereafter referred to as BION(trademark) devices, are described more fully, e.g., in U.S. Pat. Nos. 5,193,539; 5,193,540 and PCT Publications WO 98/37926; WO 98/43700 and WO 98/43701, each of which patents or publications are incorporated herein by reference. Advantageously, the BION devices are generally small enough to be implanted in a minimally invasive manner through the lumen of a needle, or a similar-sized cannula.
It has been discovered that the sharpness (rise time and/or fall time) of the pulsed modulation used with the BION device has a direct affect on the reliability of the operation of the BION device, and more particularly on the ability of the BION device to properly decode and validate commands. Disadvantageously, the sharp rise times and fall times of the pulsed modulation signals needed for reliable operation of a BION-type device have heretofore required the use of high power modulation amplifiers. High power modulation amplifiers, in turn, are not compatible with the low power requirements of an implantable medical device systems, particularly systems that include battery-powered devices. There is thus a need in the art for a low power modulation amplifier having sharp rise/fall times that may be used within an implantable device system, such as a system that uses the BION device described in the referenced patents and patent applications.
The present invention addresses the above and other needs by providing an external transmitter circuit that drives an implantable BION device, or other implantable neural stimulator, from a primary coil driven by a power amplifier. For efficient power consumption, the transmitter output circuit (which includes the primary coil driven by the power amplifier inductively coupled with an implanted coil included as part of the implantable BION device) operates as a tuned resonant circuit. Disadvantageously, when operating as a tuned resonant circuit, it is difficult to modulate the carrier signal with data having sharp rise and fall times without using a high power modulation amplifier. In order to overcome this difficulty, the present invention includes an output switch that selectively inserts a resistor in the transmitter output coil circuit in order to de-tune the resonant circuit only when data modulation is needed. Such de-tuning allows sharp rise and fall times in the data modulation without the need for using a high power modulation amplifier. Advantageously, because the data modulation is typically needed for only a very small percent of the time that a carrier signal is present, e.g., 2-4%, it is thus possible using the present invention to achieve reliable data modulation, transmission and reception within the implanted BION (or other) neural simulation device without having to use a high power modulation amplifier in the transmitter.
In accordance with one aspect of the invention, there is provided an implantable medical device, such as an implantable neural stimulator, having an implanted coil through which an externally-generated carrier signal, transmitted from an external primary coil, may be inductively received. The carrier signal is modulated with data at a low duty cycle, e.g., 4% or less, in order to transfer control data into the implantable medical device. The carrier signal provides operating power for the implantable device, either directly by continuously supplying operating power, or indirectly by supplying power on an as-needed basis to recharge a rechargeable battery housed within the implantable medical device. The carrier signal, when not modulated with data (which is typically most of the time, e.g., 96% or more), is inductively coupled to the implant coil through a high Q resonant circuit, e.g., a resonant circuit having a Q greater than about 10. The high Q resonant circuit includes the primary coil and the implant coil. Such high Q resonant circuit promotes the efficient transfer of power into the implantable medical device. A switch selectively connects a resistor in circuit relationship with the external coil to de-tune the tuned resonant circuit when the carrier signal is modulated with data (which is typically a very small percent of the time). Such de-tuning advantageously lowers of the Q of the resonant circuit to about four or less, and allows the data modulation to have sharper rise and fall times at lower power transmission levels. The sharper rise and fall times, in turn, allow more reliable data communication to occur with the implantable medical device.
One embodiment of the invention may be characterized as an implantable medical device system comprising: (1) an external power amplifier having a primary coil; (2) an implant device having an implanted coil; (3) means within the power amplifier for generating a carrier signal that is inductively coupled from the primary coil to the implanted coil through a high Q resonant circuit (Qxe2x89xa710) that includes the primary coil and the implanted coil; (4) means within the power amplifier for modulating the carrier signal with data; (5) a resistor and a switch within the power amplifier, wherein the resistor is connected to the primary coil through the switch, and wherein the resistor, when connected to the primary coil, de-tunes the high Q resonant circuit (Qxe2x89xa64); and (6) means within the power amplifier for operating the switch to de-tune the resonant circuit when the carrier signal is modulated with data, wherein the de-tuned resonant circuit allows the data modulation of the carrier signal to occur with sharper rise and fall times, which sharper rise and fall times, in turn, are more reliably detected as data within the implant device.
Another embodiment of the invention may be viewed as a low power modulation amplifier that comprises: (1) an amplifier having an output port; (2) a first capacitor (C1) connected between the output port of the amplifier and a voltage reference, e.g., ground; (3) an antenna coil (L1) connected to the output port of the amplifier through a second capacitor (C2), the second capacitor functioning as a coupling capacitor; (4) a resistor connected to the antenna coil; and (5) a switch (SW1) that switchably connects the resistor in circuit relationship with the antenna coil.
Yet another embodiment of the invention may be characterized as a method of reliably and efficiently transmitting data and power to an implantable medical device from an external power amplifier. The implantable medical device includes an implanted coil. The external power amplifier includes a primary coil. The method includes the steps of: (1) generating a carrier signal in the power amplifier; (2) inductively coupling the carrier signal from the primary coil to the implanted coil through a high Q (Qxe2x89xa710) resonant circuit that includes the primary coil and the implanted coil when only power is to be transmitted to the implantable medical device; (3) modulating the carrier signal with data when data is to be transmitted to the implantable medical device; and (4) inductively coupling the modulated carrier signal from the primary coil to the implanted coil through a low Q (Qxe2x89xa64) resonant circuit that includes the primary coil and the implanted coil. In a preferred implementation, the low Q circuit is obtained by electrically connecting a resistor to the primary coil so as to de-tune the high Q resonant circuit so that it becomes a low Q circuit.